We have studied voltage dependent membrane currents in PC12 cells in the presence and absence of nerve growth factor NGF (100 ng/ml) using the gigaohm seal technique in the whole cell voltage clamp configuration. Untreated PC12 cells in standard physiological solutions displayed a voltage dependent TEA sensitive outward membrane current which activated at potentials more depolarized that -40 mV. Typical cord conductances were 20 nS between -30 and +30 mV. The addition of TEA (1 mM) reversibly reduced the cord conductance of this outward current by 50%. In addition to the outward current described, PC12 cells grown in the presence of NGF showed a net inward current during command potentials between -40 to +20 mV from a holding potential -80 mV. The time course of the inward current activation and inactivation was rapid less than 10 msec. The net inward current reached a maximum near -20 mV decreasing with depolarization and was reversed at potentials more depolarized than +40 mV. Inactivation experiments revealed that the inward current was inactivated at holding potentials more depolarized that -40 mV. The inward current was completely abolished by 1 uM tetrodotoxin (TTX).